A conventional distributed antenna system (Distributed Antenna System, briefed as DAS) usually refers to a radio access structure. In this radio access structure, each antenna unit (Antenna Unit, briefed as AU) is installed with one or more antennas, which are placed at a regular interval in a cell and are configured to have a same cell identifier, briefed as cell ID (Identifier), all or some of the antennas are selected to serve a UE, and the selected antennas may send same or different signals. A centralized antenna system (Centralized Antenna System, briefed as CAS) is characterized in that antennas serving the UE are located at a same site (that is, a geographic position), and that the antennas also have a same cell ID.
A multi-user multiple input multiple output (Multiple Input Multiple Output) antenna system, briefed as a MIMO system, usually refers to a communication system in which a transmitter or receiver uses multiple antennas. The MIMO antenna system usually adopts distributed antennas, antennas at different sites in the system have different path losses to the UE, and transmit power of the antennas at different sites may be different. Therefore, the power received by the UE from different antennas is generally different. If a power difference caused by the path losses is not considered for the base station and the UE, system performance, such as the throughput and bit error rate, between the base station and the UE may be further degraded.
Currently, the codebook-based precoding MIMO system is a manner used by a mainstream wireless cellular system. The codebook-based precoding means that a receiver measures a radio channel from a transmitter to the receiver, to select an element from an element group, which is predefined and known by both the transmitter and the receiver, and sends a corresponding index value to the transmitter, so that the transmitter can choose to use the codebook to process a signal to be sent (the processing procedure is called precoding), and then send the signal. A set of an element group is called a precoding codebook, and one element therein is called a precoding codebook element, and the index value corresponding to the precoding codebook element is called a precoding matrix index (Precoding Matrix Index, briefed as PMI). Generally, during the process of selecting a precoding matrix index PMI, the user equipment (User Equipment, briefed as UE) needs to traverse all possible precoding codebook elements according to a specific indicator, such as a throughput rate, to select one making the indicator optimum, and sends the PMI corresponding to the selected precoding codebook element. After obtaining the PMI, the base station can obtain the corresponding precoding codebook element, and if the transmission period is short enough, it may be considered that the current downlink channel of the base station is approximate to the downlink channel measured by the UE. It is worth pointing out that the base station has a capability of independent selection and judgment. When sending a signal to the UE, the base station may use the precoding codebook element sent by the UE, or may use other precoding codebook elements, or may use precoding not based on the codebook, or even may select not to consider current scheduling before sending data to the UE, and this process is called a scheduling process of the base station. During reception, the UE may acquire which precoding codebook element is used when the signal is sent, and then perform demodulation.
Performance of a precoding-based MIMO system is usually superior to performance of a MIMO system not using precoding, and the precoding is usually based on the following assumptions: 1. Though the UE sends only the PMI corresponding to the precoding codebook, not a value of the channel, which is quantization or approximation, where the value of the channel contains all channel state information (Channel State Information, briefed as CSI), the base station can still acquire a downlink channel state through the precoding codebook to a large extent, and then process the sent signal on this basis, so that performance of UE reception is improved significantly. 2. When the UE sends the PMI corresponding to the codebook, only a small part of uplink bandwidth (called an overhead, that is, the uplink bandwidth that can transmit data to be sent to the base station by the UE is occupied by a non-data PMI) is occupied, a situation of improving downlink performance but sacrificing uplink performance will not occur.
Because the precoding-based MIMO system designs the codebook directly by using the criterion of the centralized antenna system, only the scenario where the antennas serving the UE belong to the same site (that is, a geographic position) and all the antennas serve the UE is considered for the design condition. If it is considered that the codebook designed by using the criterion of the centralized antenna system is applied to the scenario of the distributed antenna system, and a power difference caused by path losses from different antennas to the UE is taken into account, and factors to be considered in the centralized antenna system are included, all these make the size of the current codebook very large, hence increasing the feedback overhead and sacrificing the uplink performance.
If the UE sends PMIs of antennas at different sites, because the codebook elements corresponding to each sent PMI need to be further processed and combined as a large codebook element corresponding to the channel from the antenna that serves the UE to the UE, the UE needs to send a group of extra calibration values, resulting in large final feedback, hence increasing the feedback overhead and sacrificing the uplink performance. In addition, because the UE needs to send each PMI individually, a transmission sequence needs to be considered additionally, so that the scheduling of the base station can be performed smoothly, which increases complexity of the operation.